2000 Suzuki Wagon Engine not cooling enough

Till a couple of months ago my car was in perfect condition. Then on a long drive (at 36 degree Celsius) the temperature indicator went beyond half and I realized the Air conditioning cuts off. The coolant was full and the engine oil in good condition. The workshop checked for any blockage in the cooling line but could not find any.
Now due to constant overheating(current ambient temp 40 degree) the engine oil seems to be getting darker in color. Can you please suggest what could be the possible reason for this overheating. What remedy should I try.